This dataset is based on the U.S. Department of Transportation's National Bridge Inventory (NBI). Tunnels in the United States According to the HSIP Tiger Team Report, a tunnel is defined as a linear underground passageway open at both ends. Records in the NBI that are attributed as "Tunnels" were extracted by TGS and were located using a combination of ortho imagery, topographic DRGs, NAVTEQ streets, and NAVTEQ railroads. Two points were captured for each tunnel, one at each tunnel opening. A line was then created either by tracing the NAVTEQ street / railroad, or, if there was not a NAVTEQ street / railroad coincident with the tunnel, then by a straight line joining the two points. For some tunnels, the NBI contains two records, one for the road through the tunnel and one for the road on top of the tunnel (if any). In these cases, both have been captured in this dataset. Features in this dataset that are over tunnels have a [RECTYPE] of "1", while features that are in tunnels have a [RECTYPE] of "2". Presumably, this was done because both roads could be blocked if the tunnel was destroyed. In some cases, the NBI only represented a tunnel with a record of type = "1" (over). In these cases, the following rules were applied: 1) If there was no road running through the tunnel, the road on top of the tunnel was captured. For example, if a mine conveyor runs through the tunnel and a county highway runs on top of the tunnel, the county highway was captured. 2) If a road ran through the tunnel, then this road was captured and the [RECTYPE] was changed to "2". The "feature carried" and "feature intersected" fields were also changed to be consistent with the feature actually captured. According to the U.S. DOT: "Our reporting requirements do not extend to tunnels, therefore, any info we have should be considered incomplete."

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